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The set of structures needed to reason about the system, which comprises software elements, relations among them, and properties of both.

In this book, we use a definition based on the one from Software Architecture in Practice (2nd Edition) (see below). We chose it because it helps us know what to
document about an architecture. The definition emphasizes the plurality of
structures present in every software system. These structures, carefully chosen
and designed by the architect, are the key to achieving and reasoning about the
system's design goals. And those structures are the key to understanding the
architecture. Therefore, they are the focus of our approach to documenting a
software architecture.

Structures consist of elements, relations among the
elements, and the important properties of both. So documenting a structure entails
documenting those things.

The software architecture of a program or computing system is the
structure or structures of the system, which comprise software elements, the
externally visible properties of those elements, and the relationships among
them.

"Externally visible" properties refers to those assumptions
other elements can make of an element, such as its provided services,
performance characteristics, fault handling, shared resource usage, and so on.
Let us look at some of the implications of this definition in more detail.

First, architecture defines elements. The architecture
embodies information about how the elements relate to each other. This means
that architecture specifically omits certain information about elements
that does not pertain to their interaction. Thus, an architecture is foremost
an abstraction of a system that suppresses details of elements that do
not affect how they use, are used by, relate to, or interact with other
elements. In nearly all modern systems, elements interact with each other by
means of interfaces that partition details about an element into public and
private parts. Architecture is concerned with the public side of this division;
private details of elements—details having to do solely with internal
implementation—are not architectural.

Second, the definition makes clear that systems can and do
comprise more than one structure and that no one structure holds the
irrefutable claim to being the architecture. For example, all
non-trivial projects are partitioned into implementation units; these units are
given specific responsibilities, and are the basis of work assignments for
programming teams. This kind of element will comprise programs and data that
software in other implementation units can call or access, and programs and
data that are private. In large projects, the elements will almost certainly be
subdivided for assignment to subteams. This is one kind of structure often used
to describe a system. It is a very static structure, in that it focuses on the
way the system's functionality is divided up and assigned to
implementation teams.

Other structures are much more focused on the way the elements
interact with each other at runtime to carry out the system's function.
Suppose the system is to be built as a set of parallel processes. The set of
processes that will exist at runtime, the programs in the various
implementation units described previously that are strung together sequentially
to form each process, and the synchronization relations among the processes
form another kind of structure often used to describe a system.

Third, the definition implies that every software system has an
architecture because every system can be shown to be composed of elements
and relations among them. In the most trivial case, a system is itself a single
element—an uninteresting and probably non-useful architecture, but an
architecture nevertheless. Even though every system has an architecture, it
does not necessarily follow that the architecture is known to anyone.
Unfortunately, an architecture can exist independently of its description or
specification, which raises the importance of architecture documentation
and architecture reconstruction.

Fourth, the behavior of each element is part of the
architecture insofar as that behavior can be observed or discerned from the
point of view of another element. This behavior is what allows elements to
interact with each other, which is clearly part of the architecture. This does
not mean that the exact behavior and performance of every element must be
documented in all circumstances; but to the extent that an element's
behavior influences how another element must be written to interact with it or
influences the acceptability of the system as a whole, this behavior is part of
the software architecture.

Finally, the definition is indifferent as to whether the
architecture for a system is a good one or a bad one, meaning that the
architecture will allow or prevent the system from meeting its behavioral,
performance, and life-cycle requirements. Assuming that we do not accept trial
and error as the best way to choose an architecture for a system—that is,
picking an architecture at random, building the system from it, and hoping for
the best—this raises the importance of architecture evaluation.

Architecture is defined by the recommended practice as the
fundamental organization of a system, embodied in its components, their
relationships to each other and the environment, and the principles governing
its design and evolution. This definition is intended to encompass a variety
of uses of the term architecture by recognizing their underlying common
elements. Principal among these is the need to understand and control those
elements of system design that capture the system's utility, cost, and
risk. In some cases, these elements are the physical components of the system
and their relationships. In other cases, these elements are not physical, but
instead, logical components. In still other cases, these elements are enduring
principles or patterns that create enduring organizational structures. The
definition is intended to encompass these distinct, but related uses, while
encouraging more rigorous definition of what constitutes the fundamental
organization of a system within particular domains.